EP1242522B1 - Injection moulding cross joints made of nanoscalic powders - Google Patents

Abstract

The invention relates to injection moulding compounds containing a thermoplastic polymer, a wax, a nanoscalic chalcogenide-, carbide- or nitride-powder and an acid amide. The invention also relates to sintered shaped bodies produced therefrom. The sintered shaped bodies are especially useful in microtechnics, in mechanical engineering, terotechnology, construction of appliances, in micromechanics and for implants.

Description

The present invention relates to injection molding compounds with nanoscale ceramic powders, to processes for producing sintered molded articles from these injection molding compositions, to these sintered shaped articles and to their use.

Powder injection molding has become established in recent years as a ceramic molding process for ceramics and metal. Steel, cemented carbide, tungsten and cobalt alloys, interrnetallic phases, nickel-base alloy and numerous ceramic raw materials, e.g. Alumina, silicon nitride and SiC can be processed by this method. The advantages of the process lie in the achievable homogeneous powder packing, the low density gradients in moldings and above all in the ability to manufacture complex components in large numbers, cost-effective and endkontumah. Due to the possibility of being able to introduce undercuts and openings into a molded part, and due to the realizable good surfaces, the post-processing can be reduced to a low level.

Especially for ceramic microstructures a feinkömiges microstructure is mandatory, which can usually only be obtained on nanoscale ceramic powder. Therefore, an injection molding process with nanoscale ceramic powders for the formation of sintered moldings would be very interesting.

While the pressing and casting of nanoscale powders are common processes, the extrusion and injection molding of nanoscale particles are less well known. There is the difficulty that a high powder content is always mandatory, which must be realized in the still plastically deformable and processable masses. It is difficult to master the rheology, since conventional injection-molded binders with nanoscale powders become very highly viscous even at low powder contents.

In the injection molding of Keramikpulvem surface modifiers in the form of carboxylic acids, such as. For example, in the prior art for powders with particle sizes in the micron range and sub-micron range. As stearic acid, and binders or binders such as waxes, z. Paraffins having a molecular weight of 300-600 D, and polymers having a molecular weight of 10,000 D are used. However, the stearic acid surface modifier is insufficient for particle sizes in the sub-micron range to achieve sufficiently low viscosities.

Little is known about the processing of powders with particle sizes below 100 nm. Song, Evans, J. Rheology 40, 1996, 131 et seq. And Song, Evans, Ceramics International 21 (1995), 325 ff., Describe the processing of a fine powder via injection molding, in which a ZrO ₂ powder with a mean Particle diameter of 70 nm and a specific surface of 220 m ² / g is processed. To remove the binder, the so-called debinding, from the obtained after injection green body based on ZrO ₂ with a particle size of 70 nm at a molding thickness of 6 mm, a heat treatment of 195 hours, ie more than 8 days, is required. From this it can be seen that the debinding of injection-molded parts made of nanoscale powders is associated with extremely high processing periods and high energy costs.

The importance of the surface-active substances (surface modifiers) on the compatibility of the offset becomes greater as the particle size of the powders used becomes smaller. In the prior art, low molecular weight bifunctional organic molecules are usually used as surface active substances in powder injection molding. Lenk, DKG 72, 1995, 10, 636 ff. Describes the use of carboxylic acids having chain lengths of 12 to 22 carbon atoms as surface-active substances. According to Gützer, Silikattechnik 40, 1989, 2, 62 ff. Oleic acid is most often used as a surface modifier, with an optimal oleic acid occupancy with 2.5 mg / m ² surface is given. By adding oleic acid, the viscosity of the injection molding offsets is reduced, so that in this way an increase in the powder content is possible.

According to the prior art, only particle sizes with a lower limit of about 70 nm can still be processed using the surface modifiers mentioned. Below a primary particle size of 70 nm, the specific surface increases drastically and can be up to 200 m ² / g for particles of 10 nm. The resulting increased interactions with the organic processing aids and the associated high viscosity reduces the maximum possible powder content. Therefore, injection molding of such small particles according to the previous method is not possible.

A further critical factor is the debinding of the injection-molded parts. After a ceramic green body based on nanoscale particles is produced by injection molding, the critical step of debinding the molded parts follows. In this case, the entire binder must be removed from the molding, without resulting in a deformation of the molding or damage to the microstructure. The smaller the powder particles are, from which the green body is constructed, the smaller the pore size in the green body and the more difficult it is to remove the organic Prozeßhüfsmittel from the green body, without building up in the interior of the green body, a pressure that leads to a Damage to the structure leads.

US-A-5155158 relates to castable or injection moldable ceramic compositions containing sinterable powder and a polyacetal binder and a dispersing aid, which may be hydroxystearic acid. The sinterable powder typically has an average particle size in the range of 0.1 to 30 microns. The composition may contain paraffin wax.

DE-A-4212593 describes thermoplastic molding compositions containing a thermoplastic silicone resin or a thermoplastic mixture of various silicone resins and sinterable powder. In addition to the thermoplastic silicone resin, organic based thermoplastic binder such as ethylene-vinyl acetate copolymer may be contained. For the molding compound, other additives such as e.g. Called stearic acid amide and paraffin wax.

EP-A-0700881 describes binders for ceramic compositions which consist of at least one sinterable powder and at least one thermoplastic binder. The binder consists of polyamide and at least one styrene copolymer. Examples of additives include stearic acid amides and waxes.

EP-A-0501602 and US-A-4197118 describe processes for the production of sintered shaped bodies, the binder being partly removed by extraction with solvent and optionally subsequently by thermal treatment.

The object of the invention was to develop an injection molding offset with nanoscale powders, with which high powder contents can be achieved with simultaneously low viscosities in order to produce sintered molded articles of high quality. In particular, it should enable powder injection molding of nanoscale particles with sizes well below 100 nm. In addition, even in the case of very small particles, debindering without damaging the microstructure and, in particular with regard to time and energy requirements, should be economically feasible.

This object is surprisingly achieved by the injection molding composition according to the invention, the as thermoplastic polymer, a mixture of polyvinyl ether and ethylene-vinyl acetate copolymer,
at least one wax,
at least one nanoscale chalcogenide, carbide or nitride powder and
contains at least one acid amide.

In this way, it is possible to realize very high powder contents in the offset, so that in the case of very small nanoscale powders ceramic shaping is possible in the first place. This has enormous significance for the process engineering and product development that builds on it. For the first time, sintered shaped articles whose quality or structure requires the use of very small nanoscale particles can be produced by powder injection molding technology. In addition, a debinding without Gefüdemädigung even with these very small nanoscale particles is possible and can be performed easily.

The powder used is a nanoscale ceramic-forming powder. This is in particular a nanoscale chalcogenide, carbide or nitride powder. The chalcogenide powder may be an oxide, sulfide, selenide or telluride powder. Nanoscale oxide powders are particularly preferred. All powders commonly used for powder molding can be used. The powder generally contains nanoscale inorganic solid particles of semimetal or metal chalcogenides, carbides or nitrides. Examples are (optionally hydrated) oxides such as ZnO, CdO, SiO ₂ , TiO ₂ , ZrO ₂ , CeO ₂ , SnO ₂ , Al ₂ O ₃ , In ₂ O ₃ , La ₂ O ₃ , Fe ₂ O ₃ , Cu ₂ O , Ta ₂ O ₅ , Nb ₂ O ₅ , V ₂ O ₅ , MoO ₃ or WO ₃ , but also phosphates, silicates, zirconates, aluminates and stannates, sulfides such as CdS, ZnS, PbS and Ag ₂ S, selenides such as GaSe, CdSe and ZnSe, tellurides such as ZnTe or CdTe, carbides such as WC, CdC ₂ or SiC, nitrides such as BN, AIN, Si ₃ N ₄ and Ti ₃ N ₄ , corresponding mixed oxides such as metal-tin oxides, eg indium tin oxide (ITO), antimony tin oxide (ATO), fluorine-doped tin oxide (FTO) and Zn-doped Al ₂ O ₃ , luminescent pigments with Y- or Eu-containing compounds, or mixed oxides with perovskite structure such as BaTiO ₃ and PbTiO ₃ . It can be a kind of nanoscale inorganic solid particles or a mixture of different nanoscale inorganic solid particles are used.

The powders preferably contain nanoscale particles which are an oxide, hydrated oxide, chalcogenide, nitride or carbide of Si, Al, B, Zn, Cd, Ti, Zr, Ce, Sn, In, La, Fe, Cu, Ta , Nb, V, Mo or W, more preferably Si, Al, B, W, Ti and Zr. Particular preference is given to using oxides. Preferred nanoscale inorganic particulate solids are alumina, zirconia, silicon carbide, tungsten carbide and silicon nitride.

The nanoscale inorganic solid particles contained in the powder generally have an average primary particle size in the range of 1 to 300 nm or 1 to 100 nm, preferably 5 to 50 nm and particularly preferably 10 to 20 nm. The primary particles may also be present in agglomerated form, preferably they are not agglomerated or substantially non-agglomerated.

For the purpose of shaping, the starting powder is mixed with an organic binder which provides the necessary plasticization of the mixture. The injection molding composition according to the invention contains as binder components at least one mixture of polyvinyl ether and ethylene-vinyl acetate copolymer as thermoplastic polymer and at least one wax, preferably a paraffin wax.

The thermoplastic polymer used is a combination of ethylene-vinyl acetate (EVA) copolymer and polyvinyl ether. Optionally, a mixture of more than two thermoplastic polymers may be used.

Any suitable thermoplastic polymer may be used as additional optional thermoplastic polymer, especially those commonly used for powder injection molding. Examples of other useful thermoplastic polymers are polyolefins, such as polyethylene, polypropylene and poly-1-butene, vinyl polymers, such as polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polymethyl (meth) acrylate, polyacrylonitrile, polystyrene, and polyvinyl alcohol, polyamides, polyesters, polyacetals, polycarbonates, linear polyurethanes, and corresponding copolymers, with polyolefins being preferred.

As a polymer component, an EVA copolymer is used. The range X of commercially available EVA copolymers is very high in vinyl acetate content and melt index. The best results are achieved when using an EVA copolymer with a high melt index, preferably from 1 to 1000 g / 10 min, more preferably from 10 to 1000 g / 10 min, and most preferably from 100 to 1000 g / 10 min. By using EVA copolymers with a low content of vinyl acetate in the copolymer, preferably below 40%, particularly preferably below 30%, particularly low-viscosity binder systems are obtained.

Suitable EVA copolymers are e.g. Lupolen® V2520J, Lupolen V2910K, Lupolen V3510K, Lupolen V3910 DX and Lupolen V5510SX from BASF and Elvax® 450, Elvax 360, Elvax 240, Elvax 210 and Elvax 410 from DuPont, with Elvax 210 and Elvax 410 being particularly preferred.

By the mixture of EVA copolymer and a polyvinyl ether, an improvement of the offset viscosity can be achieved. The weight ratio of EVA copolymer to polyvinyl ether is preferably in the range of 0.25 to 1.50, more preferably 0.30 to 1.00, and particularly preferably 0.35 to 0.65.

Suitable polyvinyl ethers are characterized by long side chains, wherein the side chain preferably has more than 10 carbon atoms, more preferably more than 16 carbon atoms. A suitable polyvinyl ether is, for example, the Luwax® V marketed by BASF, an octadecyl polyvinyl ether.

The injection molding composition according to the invention contains at least one wax as further binder component. In this case, all conventional waxes, alone or as a mixture of two or more, can be used.

The wax may be a natural, chemically modified or synthetic wax. The natural waxes may be vegetable waxes, e.g. Candelilla wax and carnauba wax, animal waxes, e.g. Shellac wax and beeswax, mineral waxes, e.g. Ceresin, or petrochemical waxes, e.g. Petrolatum, paraffin waxes and microwaxes act. Montan ester waxes and Sasol waxes are examples of chemical modified waxes and polyalkylene waxes, and polyethylene glycol waxes are examples of synthetic waxes. Preference is given to using paraffin waxes.

Commercially available waxes are generally mixtures of n-alkanes, wherein according to the invention alkanes with chain lengths of 30 to 100 carbon atoms have proven to be very favorable and with chain lengths of 20 to 40 carbon atoms to be particularly favorable. To characterize the wax mixture is the melting range, which should preferably be between 30 and 110 ° C, more preferably between 40 and 80 ° C and more preferably between 50 and 60 ° C. As a wax component is e.g. Terhell®, type 5405 from Schümann, which has a melting range of 54-56 ° C., is particularly preferred.

Examples of waxes which can be used according to the invention are waxes from BASF: Polygen® WE5, Luwax® A, Luwax Al60, Luwax Al61, Luwax AH6, Luwax Al3, Luwax AH3, Luwax EAS1, Luwax EVA1, Luwax EVA2, Luwax AF, Luwax AF29, Luwax AF30, Luwax AF31, Luwax AF32, Luwax OA, Luwax OA3, Luwax OA4, Luwax OA5, Luwax FB and Luwax LK4 and the brands Basophob, Dewanil, Keroflux; and waxes from Shell: Paraffin Wax Grade 125/130, Paraffin Wax Grade 130/135, Paraffin Wax Grade 135/140, Paraffin Wax Grade 140/145, Paraffin Wax Grade 145/150 and Paraffin Wax Grade 150/155.

In the finished binder system, the weight ratio between the polymer component used and the wax component used (paraffin wax) is preferably in the range from 0.05 to 0.35, more preferably from 0.08 to 0.20, and particularly preferably from 0.10 to 0.13.

Particularly suitable polymer / wax mixtures according to the invention are Terhell 5405 / Elvax 210; Terhell 5405 / Lupolen VK 3510; Terhell 5405 / Lupolen 5510 Sx; Terhell 5405 / Lupolen 2520 J; Terhell 5405 / Lupolen 2910 K; Terhell 5405 / Elvax 210 and (Terhell 5405 + Luwax V) / Elvax 210. The combination of Elvax 210 with Terhell 5405 and Luvax V has proven to be particularly useful. By the polymer / wax mixtures used in the invention in general and the special mixtures mentioned in particular low-viscosity, structurally viscous flowing binder systems can be obtained, which have proven to be particularly favorable for the injection molding compositions according to the invention with nanoscale powders.

In addition to these binder constituents, the injection molding composition according to the invention contains at least one acid amide as surface modifier, which in the implementation produces the compatibility between the nonpolar waxes and polymers and the polar powder surfaces. Without wishing to be bound by theory, it is believed that at least a portion of the acid amide causes surface modification of the particles contained in the nanoscale powder.

The acid amide is generally a carboxylic acid amide derived from saturated and unsaturated carboxylic acids preferably having more than 7 and more preferably 12 to 24 carbon atoms. Acid amides of saturated carboxylic acids are preferred. Particularly preferred is a stearic acid amide. The amide can also be mono- or di-substituted on the nitrogen, for example with an alkyl group.

Examples of the preferred long-chain carboxylic acid amides are middle fatty acid amides (having 8 to 12 carbon atoms) and preferably higher fatty acid amides (having more than 12 carbon atoms), such as caprylic, pelargonic, capric, undecane, lauric, tridecane, myristic, Pentadecane, palmitic, margarine, stearic, nonadecane, arachin, beehive, lignocerin, cerotin, melissin, palmitoleic, oleic, erucic and linoleic acid amides. Further examples are N, N'-ethylenebisstearic acid amide, N- (4-hydroxyphenyl) stearic acid amide, N (-2- (diethylamino) ethyl) stearic acid amide and octadecanamide. Stearic acid amide is particularly preferred.

In general, it is possible to use all carboxylic acid amides which are derived from the carboxylic acids customarily used as surface modifiers. Surprisingly, however, it has been found according to the invention that the amides have a much more favorable influence on the processability of the injection molding compounds, e.g. in terms of viscosity, as the analog carboxylic acids show.

The injection molding compositions may optionally contain other conventional additives. Examples are plasticizers and plasticizers. The injection molding compound but preferably contains no further additives.

Preferred injection molding compounds with which the nanoscale powders can be injection-molded well contain powder contents of more than 35% by volume, preferably more than 40% by volume and more preferably more than 45% by volume. The injection molding compound preferably contains the nanoscale powder with a content of 35 to 60% by volume, in particular about 50% by volume.

The injection molding compositions contain the acid amide preferably in concentrations of 15 to 25 vol.%, Particularly preferably 18 vol.%. The wax, in particular the paraffin wax, is present in the injection molding compositions, for example in amounts of from 20 to 40% by volume, preferably from 25 to 35% by volume, particularly preferably 29% by volume.

The thermoplastic polymer mixture is preferably contained in the injection molding mixture in amounts of from 2.5 to 17% by volume, preferably from 3.5 to 10% by volume. When mixing polyvinyl ether and ethylene-vinyl acetate copolymer as a thermoplastic polymer, the ethylene-vinyl acetate copolymer is preferably present in amounts of from 0.5 to 7% by volume, preferably 0.5 to 5% by volume, more preferably 2% by volume .-%, and the polyvinyl ether preferably in amounts of 2 to 10 vol .-%, preferably 4 vol .-%, in the injection molding prior to.

The binder additives are added in a preferred embodiment in a ratio of 3-7 vol.% Polyvinyl ether (PVE), 0.5-3 vol.% EVA and 20-40 vol.% Paraffin wax (PW). The ratios are particularly preferably 4 PVE, 2 EVA, 29 PW and 18 stearic acid amide (% by volume).

Using the processing aids EVA copolymer, polyvinyl ether, paraffin wax and stearic acid amide, it was possible to produce an injection molding offset with a powder content of up to 47% by volume of nanoscale particles. For example, the mixing ratio ranges are 46.98 vol.% Al ₂ O ₃ , 1.91 vol.% EVA copolymer Elvax 210, 3.83 vol.% Polyvinyl ether Luwax V; 29.02% by volume of paraffin wax Terhell 5405 and 18.26% by volume of stearic acid amide.

The nanoscale powders are compounded with the polymers and the waxes and the carboxylic acid amide in conventional mixing or kneading. Commercially available kneading units can preferably be used at temperatures in the range from 80 to 120 ° C. for homogenization, the resulting torque preferably being between 10 and 30 Nm. Suitable devices for compounding are eg kneaders, twin-screw extruders or shear roller compactors. For the mixing process, the binder material is brought into a melt-shaped state, for example under the above-mentioned temperature effect, and then the powder and acid amide are added. The mixing or kneading process is carried out until a homogeneous mixture is achieved. The appropriate adjustment of the parameters, such as temperature and required shearing, for optimal Compounding are known in the art. By an increased shearing effect, for example, existing powder agglomerates may be broken up. After homogenization, the ceramic mass can be obtained, for example in the form of granules, which is then processed further.

The injection molding compound can be processed in conventional injection molding or transfer molding equipment to green bodies. The necessary injection pressure is between 10 and 120 bar, preferably between 20 and 90 bar and more preferably between 30 and 60 bar.

After injection molding, the binder must be removed again from the moldings, without this being damaged in their structure. The most common method of debinding green bodies is a purely thermal process. For the error-free debonding of green bodies, which are composed of very fine powders, it is necessary to choose low heating rates, so that often a period of several days is necessary for Entbindem. From this it can be seen that a one-stage thermal debindering of injection-molded parts made of nanoscale powders is associated with extremely high processing times and high energy costs. These time periods and costs can be reduced with the injection molding compositions according to the invention by removing a large part of the binder mixture via extraction, wherein the extraction can be carried out with an organic solvent or with a supercritical fluid. The injection molding compound according to the invention allows the realization of an injection molding offset in which a large part of the organic processing aids (> 50 wt .-%) is removable via an extraction process. The remaining organic processing aids give the green body the necessary stability that it needs for a more extensive sintering process, which is then carried out thermally.

From the green bodies produced with the injection molding composition according to the invention, the binder system described can therefore advantageously not only purely thermal (best conditions: 1 K / min to 450 ° C and 5 K / min to 1600 ° C) removed but large parts of the binder system can also be removed by extraction (most conveniently with n-octane at 60 ° C). The remaining binder components ensure the green strength of the components and are only thermally removed in a second step. Since the extraction can be done much faster than a thermal debinding, thus time and cost can be saved.

ZrO, molded parts from the injection molding compositions according to the invention with a thickness of 2.5 mm could be debindered within 8 hours, of which 1 hour was an extraction with n-octane at 60 ° C and 7 hours on the thermal debinding (1 K / min to 450 ° C). This corresponds to a total debinding rate of 0.156 mm / h. The Gesamtentbinderungsdauer for comparable, but purely thermally binder ZrO ₂ -Formteile (d50 = 70 nm) is according to the above-mentioned literature by Song and Evans at a Formteilchendicke of 6 mm 198 hours. This results in a debindering rate of 0.015 mm / h, ie an order of magnitude slower. Also, the extraction of the extracted binder mixture can advantageously be recovered. The extraction can be carried out with the aid of supercritical CO ₂ or with the aid of organic solvents, preferably alkanes and more preferably pentane, hexane, heptane and octane.

After debinding, the sintering of the shaped body takes place to obtain the sintered ceramic shaped body. The sintered molded article may e.g. for microtechniques, in mechanical engineering, in plant construction and in device construction, in micromechanics and for implants. Small filigree ceramic components with microstructures such as e.g. needed in micromechanics are producing with high quality and dimensional accuracy. For example, ceramic gears can be produced which are suitable for micromechanics.

The following example illustrates the invention.

EXAMPLE

The processing of a nanoscale powder of nanoparticles via injection molding is carried out initially in a commercially available kneading unit, wherein the powder content is initially set to 43% by volume. For this purpose, first 11 g of Elvax 210, 21.8 g LuwaxV and 161 g paraffin wax 5405 are mixed. Thereafter, 817 g of nanoscale Al ₂ O ₃ and 81.7 g of stearic acid amide are added. Homogenization takes place at 120 ° C. Subsequently, the offset is concentrated on a shear roller compact to achieve the final concentration of nanoparticles of 47 vol%, and granulated. Overall, this means the addition of another 200 g of nanoscale Al ₂ O ₃ , and 20 g of stearic acid. The resulting granules are then processed in an injection molding machine.

Claims (16)

1. Injection-moulding compositions comprising a thermoplastic polymer, at least one wax, at least one nanosize chalcogenide, carbide or nitride powder and at least one acid amide, characterized in that the thermoplastic polymer is a mixture of polyvinyl ether and ethylene-vinyl acetate copolymer.
2. Injection-moulding compositions according to Claim 1, characterized in that the nanosize chalcogenide powder is an oxide or sulphide powder.
3. Injection-moulding compositions according to Claim 1 or 2, characterized in that the wax is a paraffin wax.
4. Injection-moulding compositions according to any of Claims 1 to 3, characterized in that the acid amide is derived from a saturated carboxylic acid having from 12 to 24 carbon atoms.
5. Injection-moulding compositions according to Claim 4, characterized in that the acid amide is stearamide.
6. Injection-moulding compositions according to any of Claims 1 to 5, characterized in that the acid amide is present in concentrations of from 15 to 25% by volume, preferably 18% by volume.
7. Injection-moulding compositions according to any of Claims 1 to 6, characterized in that the ethylene-vinyl acetate copolymer is present in amounts of from 1 to 5% by volume, preferably 2% by volume.
8. Injection-moulding compositions according to any of Claims 1 to 7, characterized in that the polyvinyl ether is present in amounts of from 2 to 10% by volume, preferably 4% by volume.
9. Injection-moulding compositions according to any of Claims 1 to 8, characterized in that the wax is present in amounts of from 25 to 35% by volume, preferably 29% by volume.
10. Injection-moulding compositions according to any of Claims 1 to 9, characterized in that the powder content is from 35 to 60% by volume, preferably 50% by volume.
11. Injection-moulding compositions according to any of Claims 1 to 10, characterized in that the resulting torques at a powder content of 45% by volume are below 20 Nm.
12. Process for producing injection-moulding compositions according to any of Claims 1 to 11, characterized in that at least one nanosize chalcogenide, carbide or nitride powder and at least one acid amide are compounded with a mixture of at least one thermoplastic polymer comprising a mixture of polyvinyl ether and ethylene-vinyl acetate copolymer and at least one wax and the injection-moulding composition is obtained in the form of a pelletized material.
13. Process for producing a sintered shaped body, in which an injection moulding according to any of Claims 1 to 11 is injection moulded in an injection-moulding unit or a transfer-moulding unit and the green body obtained is subjected to binder removal and sintered.
14. Process according to Claim 13, characterized in that the binder is firstly partly removed by extraction and subsequently by heat treatment.
15. Sintered shaped body obtainable by a process according to Claim 13 or 14.
16. Use of the sintered shaped bodies according to Claim 15 for microtechniques, in machine construction, in plant construction and in instrument construction, in micromechanics and for implants.